Transistor noise measuring circuit

ABSTRACT

A plurality of serially connected, forward-biased diodes and a resistor supply base current from a voltage supply to a transistor under test. Bias circuit impedance is thus maintained high so that the transistor&#39;&#39;s output noise is primarily due to the transistor&#39;&#39;s equivalent input current noise and not to the transistor&#39;&#39;s equivalent input voltage noise nor to the noise generated in the bias network.

United States Patent Inventor Knud Knudsen Los Altos Hills, Call. Appl.No. 1,582 Filed Jan. 9, 1970 Patented Nov. 16, 1971 AssigneeHewlett-Packard Company Palo Alto, Calif. Priority Jan. 24, 1969 Japan44/0481 1 TRANSISTOR NOISE MEASURING CIRCUIT 1 Claim, 3 [having Figs.

U.S. (I 324/158 T Int. (1 G0lr 31/22 Field oISearcla 324/158,

[56] References Cited UNTI'ED STATES PATENTS 3,041,537 6/1962 Cagleetal. 324/158T 2,935,684 5/1960 Lanning 324/158T Primary Examiner-EliLieberman Attorney-A. C. Smith ABSTRACT: A plurality of seriallyconnected, forwarrbbiased diodes and a resistor supply base current froma voltage supply to a transistor under test. Bias circuit impedance isthus maintained high so that the transistor's output noise is primarilydue to the transistor's equivalent input current noise and not to thetransistors equivalent input voltage noise nor to the noise generated inthe bias network.

PATENTEDHUV 1s |97l PRIOR ART BIO TRANSISTOR NOISE MEASURING CIRCUITBACKGROUND OF THE INVENTION It is convenient in analyzing circuitdesigns and circuit operations if noise produced by a transistor inoperating condition is represented by means of two noise sourcesconnected to the input terminals. As an example of this way ofrepresentation FIG. 1 shows a common emitter connected bipolartransistor with all internal noise sources represented by the noisevoltage source, e and the noise current source, 1' In order to measurethe noise contribution from i alone one has to measure the noise at thetransistor's output terminals while the input terminals are opencircuited. But in order actually to make a transistor operate at aprescribed condition, a bias source must be inserted into thebase-emitter circuit of the transistor under test and a prescribed basecurrent must be supplied to the base. it is impossible, therefore, toconduct measurement ideally under an open base condition. The open basecondition must therefore be approximated, and two requirements on thenetwork which feeds the base current to the transistor under test mustbe met: (1) the network must have an impedance which is much larger thanthe highest expected value of e Ii of the transistor under test for thespecified bias condition, and (2) it must generate much less noie thanthe minimum expected value of i of the transistor under test for thespecified bias condition. in FIG. 2 where a resistor 5 is used to feedthe base current to a bipolar transistor under test above requirementsare met if the resistor value is much larger than (2KI)/(q1,)+r,,'=(50mv.)/(l )+r where k is Boltzmann's constant, T the absolute temperature,q the electron charge, I, the DC base current and r, the intrinsic baseresistance. Therefore, the voltage drop developed across resistor 5 by X1,, must be much larger than 50 mv. Let the minimum value of saidvoltage drop be, said 2 volts. Clearly for the reason of practicallimitation on the supply 6 a single value of resistor 5 is not feasiblewhen the value of the base bias current is expected to vary considerablymore than one order of magnitude.

SUMMARY OF THE INVENTION in the present invention the resistor 5 in FIG.2 is replaced by a series connection of a plurality of diodes and aresistor. The base current passes through the diodes in their forwardconduction direction. Using an appropriate number of suitable diodes andan appropriate resistor value the series connection of diodes andresistor will have an impedance large enough that for measurement ofnoise from a bipolar transistor the contribution from e will be muchsmaller than the contribution from i for a wide range of base current.Also the noise current generated in the series connection will be muchsmaller than the minimum possible value of i for a wide range of basecurrent.

The range of voltage variation across the series combination will bemuch smaller than the range of base current because the voltage dropacross the diodes is a logarithmic function of current. Therefore thelimitation encountered when using only a resistor 5 as in lFlG. i hasbeen overcome.

BRllElF DESCRHPTION OF THE DRAWING FIG. l is an equivalent circuitrelated to noise of transistors;

FIG. 2 is a schematic diagram of the structure of a conventional andhnown circuit for measuring the input current noise, 1' of a transistor;and

FIG. 3 is a schematic diagram of a preferred embodiment of the presentinvention. in all the figures, corresponding components are designatedwith identical symbols.

DESCRIPTION OF THE PREFERRED EMBODHMENT In reference to the preferredembodiment of the present invention, a row of diodes consisting of anadequate number of diodes, and a resistor are serially connected betweenthe bias voltage source and the base of the transistor under test. FIG.3 which can be found to be less than or equal to indicates the outlineof a transistor noise measuring circuit embodying the present invention.According to the figure, 2 represents a transistor-operating circuit forsetting specified operating condition of the transistor under test 4. inthis circuit, an NPN-type transistor 4 is connected in the commonemitter configuration. 6 denotes a bias voltage source, and i0 denotes adiode row consisting of an adequate number of diode elements 11 seriallyconnected with a resistor 12. The connection polarity of all diodes isidentical, and base current I supplied from the bias voltage source tothe base is conducted in forward direction through the diodes. Further,in reference to the Figures, 21 is the connection terminal for anexternal noise signal source or calibration signal source (neither isshown in the figures), and 3 is a coupling capacitor between thisterminal and the base electrode, and for the purpose of measuring noisecurrent source 1' the capacitance of capacitor 3 should be so small thatthe internal impedance of the external signal source will not affect themeasurement of the noise current source, and also that it may beregarded as a constant. 7 indicates the collector load resistor, and 8represents amplifiers installed for the purpose of amplifying the noiseproduced by transistor circuit 2, and to its output 9 are connected thenoise-measuring apparatus (not shown) for measuring the amplified noise.

in reference to the present preferred embodiment, all the diodesnumbered 11 are so connected that the base current I, supplied by biasvoltage source 6 to the base flows in the forward direction of thediodes. lfthe number of diodes in the series connection is N and if thebase current at all times is much larger than the saturation current ofthe diodes, the impedance of the series connection can be written asZOUT=N7ZI;Z1+RB where m depends on the diodes used, 15 m S 2, and R isthe resistor 12 in FIG. 3. we a L 12...

ou'r N qIB Rads To insure that in the output noise of the transistor thecontribution from e is much smaller than that from i we require 0w -fi1v2 For a bipolar transistor the highest possible value of Q 2 zkl r 'mm(a...) T

= ll Q n+1 21am, 5+1

a l; qI B+1 here B is the common emitter low-frequency current gain ofthe transistor and B is the common emitter DC current gain. it

we find that Z e /iii N l and R,, r,'. For n, we use the largestexpected value for all transistors to be measured. If the diodesgenerate only shot noise, the mean squared voltage across N diodes andresistor R, is:

2 1 -($f) -N+4ktTR (V /Hz) the corresponding mean squared current is:

lcT 2 2 4k qIn( qIB N+ TR 2 (A /Hz) Nun.)

The minimum value of for a bipolar transistor is: 1 Claim! 2 I B 1 l. Acircuit for measuring the equivalent input current noise q i=2qI of abipolar transistor under test having input and output B signal ports,the circuit comprising:

first bias supply means for supplying a selected value of bill so bychoosing N sufficiently large the noise current generated signal to theinput p of a transistor under test; by the series combination of diodesand resistor will be much at least approximately diodes and a resistorserially consmaller than the minimum value of i nected in commonconduction direction between said Thus, the series combination of diodesand resistor used to first bias supply means and the input port of atransistor supply the base current permits measurement of 1' of abipolar under test, said common conduction direction forward transistorfor a wide range of base current, and the range of biasing said diodesfor bias signal supplied therethrough voltage drop across the seriescombination will be comto the in ut port of the transistor under test;pressed. in practice, N is typically 20 to 30 and this limits the 1second bias signal supply means connected to the output the reedlhg of Nto approximately 3 to 5 Pefeehh port of the transistor under test forsupplying a selected 6591,! we??? "le ee of h resistor qnset qinsfivalue ofbi'as signal thereto; and with the plurality efdiodes is yp yabout times the circuit means coupled to the output port of thetransistor imum expected values of the n, of the transistors to berneaunder test f l ing an output repre entative of the Tsured and thevalue of this resistor is usually greater than apnoise i al du ed by thetransistor under test. proximately 1 kilohm. 20 g UNETED STA'EES PATENT()FFECE CERTEFICATE F CGRECHQN Patent No. 3.621.394 Dated Nnvember 161971 Inventofls) Knud Knudsen It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 1, line 30, "(2K1)" should read (2161') Signed and sealed this23rd day of May @912.

(SEAL) ttest:

EDE J'AI-QD MELETCF-E JR. ROBERT C OTTSGIiQLK Attestlng GfflcerCommissioner of Patents FORM PO'WSO USCOMM-DC 60376-P69 3 also 0 i US,GDVERNHENT :RINTINC OFFCZ W69 D-BEu-Mh

1. A circuit for measuring the equivalent input current noise of abipolar transistor under test having input and output signal ports, thecircuit comprising: first bias supply means for supplying a selectedvalue of bias signal to the input port of a transistor under test; atleast approximately 20 diodes and a resistor serially connected incommon conduction direction between said first bias supply means and theinput port of a transistor under test, said common conduction directionforward biasing said diodes for bias signal supplied therethrough to theinput port of the transistor under test; second bias signal supply meansconnected to the output port of the transistor under test for supplyinga selected value of bias signal thereto; and circuit means coupled tothe output port of the transistor under test for supplying an outputrepresentative of the noise signal produced by the transistor undertest.